1. Field of the Invention
This invention relates to a fluorescent composition and a fluorescent luminous device, and more particularly to a fluorescent composition capable of emitting light of various luminous colors under electron or ultraviolet excitation and a fluorescent luminous device utilizing such a phosphor composition.
2. Description of the Prior Art
A fluorescent composition is generally adapted to carry out emission due to excitation by external energy. The fluorescent composition is classified depending on energy for excitation. A fluorescent composition excited by electrons is called an electron excitation fluorescent composition, which includes a fluorescent composition for a CRT which exhibits luminance under electron excitation by an acceleration voltage of several ten KV and a fluorescent composition for a fluorescent display device which exhibits luminance by an acceleration voltage of several ten V.
A fluorescent composition excited by ultraviolet rays is called an ultraviolet excitation fluorescent composition, which includes a fluorescent composition for a fluorescent lamp which emits light of various luminous colors due to excitation by ultraviolet rays radiated due to discharge of a vapor of mercury.
Now, a fluorescent composition for a fluorescent display device will be described.
A fluorescent composition for a fluorescent display device is also called a low electron excitation fluorescent composition, because it exhibits luminance under excitation by a low acceleration voltage of several ten volts.
Of the fluorescent composition, a ZnO:Zn phosphor of a green luminous color can be energized by a luminance threshold voltage of only about 1 to 2 volts, with sufficient luminescence for display being obtained at an anode voltage of 10 to 20 volts. Because of such characteristics, the ZnO:Zn phosphor is superior as a low velocity electron excitation fluorescent composition. However, color emission from the ZnO:Zn phosphor is limited to the color green, thus, a sulfide system phosphor is extensively used to obtain luminous colors other than green.
The sulfide system phosphor includes, for example, ZnS:(Zn) (blue luminous color), ZnS:Mn (yellow to orange-colored), ZnS:Ag (blue), ZnS:Ag,Al (blue), ZnS:Au,Al (yellow green), (Zn.sub.0.9 Cd.sub.0.1)S:Au,Al (yellow), (Zn.sub.0.8 Cd.sub.0.2)S:Au,Al (orange-colored), Y.sub.2 O.sub.2 S:Eu (red) and the like. Unfortunately, the sulfide system phosphors each have high resistance, so that a conductive material such as In.sub.2 O.sub.3, SnO.sub.2 or the like is incorporated in the phosphor to decrease the resistance. However, such a conductive material blocks luminance of the phosphor to decrease luminous efficiency of the phosphor, because it is not a luminous material.
Also, the sulfide system phosphor is decomposed upon excitation by electron energy to scatter sulfide-containing gas because it contains sulfur. When the sulfide-containing gas is adhered to a filamentary cathode, it reacts with oxide of alkaline earth metal on a surface of the cathode to poison the surface, resulting in emission characteristics of the cathode being deteriorated as known to those skilled in the art. It is also known that such a phenomenon remarkably appears particularly when density of electrons exciting the phosphor is high.
In order to solve such a problem, it is proposed to increase luminous efficiency of the phosphor so that it may exhibit satisfied luminance even when density of electrons is low. More particularly, electrons of low density are impinged on the sulfide system phosphor to decrease energy of the electrons, to thereby prevent decomposition of the phosphor.
However, the conventional sulfide system phosphor contains a conductive material which does not contribute to luminance, so that a part of electrons flows through the conductive material to an anode conductor. This causes the part of the electrons to constitute an invalid current, resulting in luminous efficiency of the phosphor being deteriorated. Also, a decrease in density of electrons correspondingly decreases electrons exciting the phosphor, to thereby reduce luminance of the phosphor.
Further, a fluorescent composition of a non-sulfide system is also proposed, however, such a composition available fails to exhibit satisfactory luminous color, luminous efficiency and life.
As one of color fluorescent compositions known other than the sulfide system phosphor, a composite oxide phosphor of a gallate system is disclosed in Japanese Patent Publication No. 31236/1985. The phosphor has a composition indicated by a formula A(Zn.sub.1-x,Mg.sub.x)O.multidot.Ga.sub.2 O.sub.3 (0.6.ltoreq.A.ltoreq.1.2 and 0.ltoreq.x.ltoreq.0.5). The luminous color is blue at x=0 and is shifted to a long wavelength side to approach the color green as x goes away from 0. However, this causes its luminance threshold voltage to be increased.
Also, the A(Zn.sub.1-x,Mg.sub.x)O.multidot.Ga.sub.2 O.sub.3 phosphor is decreased in luminescence to a degree sufficient to fail to permit it to be put to practical use. For example, it is described in the above-noted Japanese publication that ZnO.multidot.Ga.sub.2 O.sub.3 corresponding to A(Zn.sub.1-x,Mg.sub.x)O.multidot.Ga.sub.2 O.sub.3 of A=1 and x=0 exhibits luminescence in the order of 4ft-L with an anode voltage of 80V and a cathode voltage of 0.6 V. Also, the Japanese publication teaches that a (Zn.sub.0.7 Mg.sub.0.3)O.multidot.Ga.sub.2 O.sub.3) phosphor having MgO incorporated therein to obtain A=1 and x=0.3 has a luminous wavelength shifted to a long wavelength side, so that an increase in luminescence may be limited to 8ft-L at most under the same driving conditions of an anode voltage of 80 V and a cathode voltage of 0.6 V. Thus, it will be noted that the phosphor fails to exhibit luminescence sufficient to permit it to be put to practical use.
There is known a fluorescent composition which exhibits luminance under ultraviolet excitation other than low velocity electron excitation and carries out emission of various luminous colors extending from blue to red with good efficiency. Ultraviolet excitation does not cause decomposition of a fluorescent composition as compared with low velocity electron excitation, because the former utilizes light.